124 



ELECTRO-MAGNETISM. 



was more rapid when it passed through dense than through rarefied air ; and, 

 in this case, the conducting medium, or chain of aeriform particles, was much 

 shorter.* 



While these investigations were proceeding in France and England, the dis- 

 coveries to which they led conducted a German philosopher to the invention 

 of an instrument of physical inquiry of surpassing beauty and utility, and 

 equalled in importance by none which had appeared since the balance of 

 torsion. 



The multiplier, or, as it has been called in England, the galvanometer, in- 

 vented by Schweiger, is an instrument by which the presence of an electric 

 current is detected, and its intensity measured. It is based upon the fact, that 

 a wire through which a current passes will have a tendency to turn a magnetic 

 needle at right angles to it. By this beautiful instrument the most feeble cur- 

 rents may be made manifest, and their intensities compared. It is various in 

 its construction, according to the nature and power of the electric currents in- 

 tended to be observed, but generally consists of a rectangular frame of wood, 

 round two parallel sides of which a copper wire, lapped with silk, is rolled, so 

 that the coils of wire shall be close beside each other, and parallel in their gen- 

 eral direction to the other two sides of the frame. Within the frame, and be- 

 tween the two surfaces formed by the coils of wire, is suspended a magnetic 

 needle. If the frame be so placed that the needle, when at rest, shall be par- 

 allel to the coils of wire, these coils will be parallel to the magnetic meridian. 

 Matters being thus disposed, let the extremities of the wire be put in connex- 

 ion with the poles of a Voltaic pile. The current passing through the wire 

 will act upon the needle, and each coil of the wire will affect it as a separate 

 current, so that the total effect will be in proportion to the number of coils. If 

 the current in the lower coils moved in the same direction as the upper, it 

 would have a contrary effect on the needle ; but, by the manner in which the 

 wire is carried round the frame, the systems of inferior currents are contrary 

 in their direction to the superior currents, and they have, consequently, the 

 same effect on the needle. If the effect of the current thus multiplied be suffi- 

 cient, the effects of the earth's magnetism will be overcome, and the needle 

 will be turned at right angles to the wires, and, consequently, will take the 

 direction of magnetic east and west ; but if the force of the current be insuffi- 

 cient for this, the needle will be deflected at some definite angle with the mag- 

 netic meridian, the magnitude of which angle will supply the means of estima- 

 ting the force of' the current. 



It is evident that the sensibility of this instrument will be augmented in pro- 

 portion as the magnetism of the needle is enfeebled, and the number of coils of 

 wire augmented. • 



The direction of the current is indicated by the direction in which the de- 

 flection of the needle takes place. If the north pole of the needle be deflected 

 toward the east when the current passes in one direction through the wire of 

 the multiplier, it will be equally deflected toward the west when the same cur- 

 rent is reversed. 



When Ampere had demonstrated the reciprocal action of electric currents 

 on each other, and on magnets, he showed that the terrestrial globe exerted an 

 influence on magnets freely suspended, and on electric currents transmitted 

 through wires so supported as to be capable of obeying any forces exerted upon 

 them, identical in all respects with the influence which a sphere would exert 

 round which a wire coiled so that its coils shall nearly coincide with the paral- 

 lels of latitude, through which wire an electric current is transmitted, running 

 continually from east to west, or contrary to the diurnal motion of the earth ; 



* Phil. Trans., 1821 ; Davy's works, vol. vi, p. 232. 



